Gltf-model export from blender. Materials are missing in a-frame
glTF 2.0 specification only supports exporting PBR materials based on a metallic-roughness model.

Material support for Blender's glTF exporter only works for Cycles render materials at the moment.

And even then it requires a specific node setup to be used, as mentioned in the user documentation, through the use of a Node Group designed for this purpose. The node group is available for download from the repository.

For future versions, EEVEE material node tree support is planned, using the Principled BSDF Shader, but it is unavailable at the moment.

Also see .fbx export why there are no materials or textures?